Current glass tempering machines employ what are known as oscillating roller furnaces in which glass is heated mainly by radiation. In the tempering process the temperature of the glass is increased above the softening point of glass in order to enable the glass to be tempered. This temperature is between 610 and 625.degree. C. depending on the thickness of the glass. The glass is then cooled at desired speed typically using forced convection whereby air jets are blown at the glass from above and from below. This method enables high heat-transfer coefficients, necessary when thin glass is concerned in order to achieve a sufficient temperature difference between the surface and centre of the glass. Examples of oscillating roller furnaces are disclosed in Fl patents 83,072 and 86,407.
When heating glass in a tempering furnace, where the glass is oscillated in a reciprocating manner upon rollers during the entire heating process and the glass is heated with resistors located above and below the glass, heat transfer to the glass is difficult to control. A reason for this is that heat transfer from massive rollers to glass, for example, is predominant particularly at the initial heating stage. In this case t e lower surface of the glass is subjected to a greater thermal current (than) above, even if the heat transfer of the upper surface had been intensified e.g. by utilizing forced convection. This makes the edges of the glass bend upwards and the contact surface between the glass and the rollers becoming quite indefinite. Furthermore, the surface pressure at the point of contact where the glass touches the roller becomes high enough to subject the glass to optical faults, i.e. white marks and scratches, breaking the surface of the glass. Furthermore, the conditions in the furnace change during the heating period. The temperature of the glass changes relative to time and particularly the heat transfer from the rollers diminishes as the temperature of the glass approaches the temperature of the rollers and as the temperature of the rollers falls at the initial stage of the heating period when the thermal current taken up by the glass is at its highest. This causes the problem of keeping the heating of the upper and lower parts of the furnace equalized during the entire heating period.
At the start of heating, a power equal to the combined power of the resistors is connected to the furnace, i.e. each resistor is switched on at maximum heating power. In this case it is impossible to adjust the furnace, and it is impossible to control the actual relative power of the upper and lower sides because of resistor delays. In this case unevenly arranged glass loads in the furnace cause the furnace to become cooled in places comprising most glass, even though the need for heating is greatest in these places.